Methods for enriching pluripotent stem cell-derived cardiomyocyte progenitor cells and cardiomyocyte cells based on sirpa expression

ABSTRACT

The present invention relates to in vitro methods of enriching populations of human pluripotent stem cells that are induced to differentiate to cardiomyocyte progenitor cells and cardiomyocyte cells. The cell populations can be enriched by isolating cells that express SIRPA. The invention also related to in vitro-enriched populations of cardiomyocyte cells and cardiomyocyte progenitor cells obtained from populations of pluripotent stem.

RELATED APPLICATIONS

The application claims priority from U.S. Provisional Patent ApplicationNo. 61/377,665 filed on Aug. 27, 2010.

FIELD OF THE INVENTION

The present invention relates to methods for enriching pluripotent stemcell-derived cardiomyocyte progenitor cells and cardiomyocyte cellsbased on SIRPA expression.

BACKGROUND Of THE INVENTION

The potential of human embryonic (hESCs) and induced pluripotent stemcells (hiPSCs) to generate cardiovascular cells in culture provides apowerful model system for investigating cellular interactions andmolecular regulators that govern the specification, commitment andmaturation of these lineages, as well as a unique and unlimited sourceof human cardiomyocytes for drug testing and regenerative medicinestrategies¹⁻⁴. Translating this remarkable potential into practice is,however, dependent on technologies that enable the reproduciblegeneration of highly enriched populations of cardiomyocytes, ascontaminating cell types could impact drug responses and otherfunctional properties in vitro and increase the risk for abnormal growthand teratoma formation following transplantation in vivo⁵. When inducedunder optimal cardiac conditions, human pluripotent stem cells (hPSCs)will efficiently differentiate to generate mixed cardiovascularpopulations, including cardiomyocytes, smooth muscle cells, fibroblastsand endothelial cells³. While cardiomyocytes can represent up to 70% ofthe population for any given hPSC line, the efficiency of generatingthis lineage does vary considerably between different stem cell lines.Further manipulation of induction conditions has not yet yieldedstrategies for the generation of pure populations of cardiomyocytes froma broad range of hPSC lines.

To enrich for cardiomyocytes from the differentiation cultures,cardiomyocyte-specific fluorescent reporters or drug selectable elementshave been introduced into hPSCs⁶⁻⁸. Following differentiation,cardiomyocytes can be enriched either by fluorescent-activated cellsorting (FACS) or the addition of appropriate selection drugs. Althoughthese strategies do allow for the generation of enriched cardiomyocytepopulations, they suffer from a major drawback as a reporter vector mustbe introduced into each hPSC line used, resulting in geneticallymodified cardiomyocytes, thus reducing their utility for clinicalapplications. In a more recent study, Hattori et al. demonstrated thatit was possible to isolate cardiomyocytes by FACS, based on their highmitochondrial content⁶. While this approach appears to be useful forisolating mature cardiomyocytes, cells with fewer mitochondria, such asimmature hPSC-derived cardiomyocytes, may be more difficult todistinguish from other cell types.

SUMMARY OF THE INVENTION

In an aspect, there is provided a method of enriching a population ofcells for cardiomyocyte cells and cardiomyocyte progenitor cellscomprising providing the population at cells from which cardiomyocytecells and cardiomyocyte progenitor cells are to be isolated; andisolating from the population, cells expressing SIRPA; wherein thepopulation of cells comprises a population of human pluripotent stemcells induced to differentiate into cardiomyocyte cells andcardiomyocyte progenitor cells.

In a further aspect, there is provided an enriched population ofcardiomyocyte cells and cardiomyocyte progenitor cells obtained usingany one of the methods described herein.

In a further aspect, there is provided an isolated population of cellsenriched for cardiomyocyte cells and cardiomyocyte progenitor cells,wherein the population of cells comprises at least 60%, preferably atleast 90%, cardiomyocyte cells and cardiomyocyte progenitor cells.

In a further aspect, there is provided the use of SIRPA for isolatingcardiomyocyte cells and cardiomyocyte progenitor cells from a populationof cells, wherein the population of cells comprise a population of humanpluripotent stem cells induced to differentiate into cardiomyocyte cellsand cardiomyocyte progenitor cells.

In a further aspect, there is provided a method of depleting apopulation of cells for cardiomyocyte cells and cardiomyocyte progenitorcells comprising: providing the population of cells from whichcardiomyocyte cells and cardiomyocyte progenitor cells are to bedepleted; and depleting from the population, cells expressing SIRPA;wherein the population of cells comprises a population of humanpluripotent stem cells induced to differentiate into cardiomyocytecells, cardiomyocyte progenitor cells, and non-cardiomyocytes.

In a further aspect, there is provided a method of enriching apopulation of cells for cardiomyocyte cells and cardiomyocyte progenitorcells comprising: providing the population of cells from whichcardiomyocyte cells and cardiomyocyte progenitor cells are to beisolated; and depleting from the population, cells expressing at leastone of CD90, CD31, CD140B and CD49A; wherein the population of cellscomprise a population of human pluripotent stem cells induced todifferentiate into cardiomyocyte cells, and cardiomyocyte progenitorcells, and non-cardiomyocytes.

DESCRIPTION OF THE DRAWINGS

Embodiments of the invention may best be understood by referring to thefollowing description and accompanying drawings. In the drawings:

FIG. 1 shows specification of the cardiovascular lineage from hESCs. (a)Outline of the protocol used to differentiate hESCs to the cardiaclineage (modified from Yang et al., 2008). (b) Quantitative PCR (QPCR)analysis of BRACHURY (T), MESP1, ISLET1 (ISL1), NKX2-5, MYH6 (αMHC),MYH7 (βMHC), MYL2 (MLC2v), MYL7 (MLC2a), NEUROD1 and FOXA2 inHES2-derived embryoid bodies (EBs) at different stages duringdifferentiation. Day 0, hES cells; LV, human fetal left ventricle; LA,human fetal left atria; AH, human adult heart, Ed, hESC-derivedendoderm¹³. Bars represent mean±standard error of the mean, n=3.

FIG. 2 shows expression of the cell surface receptor SIRPA during hESCdifferentiation. (a) Flow cytometric analysis of SIRPA (SIRPA) on EBsderived from NKX2-5-GFP hESCs. (b) Expression of SIRPA on HES2-derivedEB populations at the indicated times. (c) RT-qPCR analysis ofexpression of SIRPA and its ligand CD47 in HES2-derived EBs at differenttimes of differentiation. Day 0, ES cells; LV, human fetal leftventricle; LA, human fetal left atrial; AH, human adult heart. Barsrepresent mean±standard error of the mean, n=4. (d) Immunostaining forSIRPA and cardiac Troponin I (cTNI) on cardiac monolayer cultures.Monolayers were generated from d20 HES2-derived EBs.

FIG. 3 shows enrichment of cardiomyocytes from hESC-derived cultures bycell sorting based on SIRPA expression. (a) Flow cytometric analysis ofSIRPA expression in EBs at d8, d12 and d20 of differentiation.Fluorescent-activated cell sorting (FACS) for SIRPA was performed at d8,d12 and d20. The presort (PS), SIRPA⁺ and SIRPA⁻ fractions from eachtime point were analyzed for cardiac Troponin T (cTNT) expression byintracellular flow cytometry. The frequency of cTNT⁺ cells at d8, d12and d20 was significantly higher in the SIRPA⁺ fraction (day8:95.2%±1.9, day12: 94.4±1.7, day20: 89.6±3.6), compared to SIRPA⁻ cells(day8: 13.0±2.1, day12: 14.3±3.9, day20: 15.7±6.0). (b) Averageenrichment of cTNT⁺ cells from 3 different cell separation experiments.Bars represent standard error of the mean. Asterisks indicatestatistical significance as determined by student's t-test, ***(p≤0.001) (c) QPCR analysis of PS, SIRPA⁺ and SIRPA⁻ cells. Expressionof SIRPA, NKX2-5, MYH6, MYH7 and MYL7 was significantly higher in theSIRPA⁺ fraction compared to SIRPA⁻ fraction at all stages analyzed (d8,d12 and d20). Expression of markers for the non-cardiac lineages (PECAMand DDR2) segregated to the SIRPA⁻ fraction. Bars representmean±standard error of the mean. Asterisks indicate statisticalsignificance as determined by student's t-test, * (p≤0.05), ** (p≤0.01),*** (p≤0.001), n=3. (d) Immunostaining of cardiac Troponin I (cTNI) onmonolayer cultures generated from PS, SIRPA⁺ and SIRPA⁻ cells sorted atday20.

FIG. 4 shows enrichment of cardiomyocytes from hiPSC-derived cultures bycell sorting based on SIRPA expression. (a) Flow cytometric analysis ofSIRPA expression at d20 of differentiation on 38-2 and MSC-iPS1hiPSC-derived cells. Fluorescent-activated cell sorting (FACS) for SIRPAwas performed at d20 and the presort (PS), SIRPA⁺ and SIRPA⁻ fractionswere analyzed for cardiac Troponin T (cTNT) expression by intracellularflow cytometry. (b) The frequency of cTNT⁺ cells was significantlyhigher in the SIRPA⁺ fraction of both hiPSC-derived cultures (MSC-iPS1:67.0±3.6, 38-2: 71.4±3.8), compared to SIRPA⁻ cells (MSC-iPS1: 4.9±2.1,38-2: 6.2±0.9). Bars represent mean±standard error of the mean.Asterisks indicate statistical significance as determined by student'st-test, ** (p≤0.01), *** (p≤0.001), n=3. (c) QPCR analysis of PS, SIRPA⁺and SIRPA⁻ cells derived form MSC-iPS1 and 38-2 hiPSCs after cellsorting at d20. Expression of markers specific for the cardiac lineage(SIRPA, NKX2-5, MYH6, MYH7, MYL2 and MYL7) was significantly higher inthe SIRPA⁺ compared to the SIRPA⁻ fraction. Expression of markers forthe non-cardiac lineages (DDR2, PDGFRB and NEUROD1) segregated to theSIRPA⁻ fraction and the PS cells. Bars represent mean±standard error ofthe mean. Asterisks indicate statistical significance as determined bystudent's t-test. * (p≤0.05). ** (p≤0.01), *** (p≤0.001), n=5.

FIG. 5 shows expression of SIRPA on human fetal cardiomyocytes and inadult human heart. (a) RT-qPCR analysis for SIRPA in human fetal hearttissue and adult heart. LV, left ventricle; RV, right ventricle; AP,Apex; LA, left atria; RA, right atria, AVJ, atrioventricular junction;HEK, human embryonic kidney cells; AH, adult heart; day 0, hES cells;d20, day20 of cardiac differentiation, RT, reverse transcriptasecontrol. Bars represent mean±standard error of the mean, n=6. (b)Immunostaining for SIRPA (green) on human fetal ventricular cells andstaining with Mito Tracker Rod (red, accumulates in the mitochondrialmatrix) and DAPI (blue, nuclear dye). (c) Flow cytometric analysis forSIRPA on human fetal heart tissue. (d) intracellular flow cytometricanalysis for cTNT on human fetal heart tissue.

FIG. 6 shows the utilization of SIRPA to predict cardiac differentiationefficiency. (a) Day5 KDR/PDGFRA flow cytometry profiles of cardiacdifferentiation cultures induced with varying combinations of Activin A(ACTA0, 3, 6, 9 ng/ml) and BMP4 (10, 30 ng/ml). The KDR⁺PDGFRB⁺population has been shown to contain the cardiac mesoderm cells². (b)Day 9 SIRPA flow cytometric analysis expression profiles of the culturesdescribed in (a). (c) Day 20 cTNT profiles (intracellular flowcytometric analysis) of the cultures described in (a). (d)Quantification of a-c. Close correlation of expression of SIRPA at day 9(green dots) and cTNT expression at day20 (red rhombuses) illustratesthe predictive potential of SIRPA for cardiac differentiationefficiency.

FIG. 7 shows enrichment of cardiomyocytes through negative selection.(a) Flow cytometric analysis of markers specifically expressed onnon-myocyte (SIRPA-negative) cells in day 20 differentiation cultures(HES2). (b) Fluorescent activated cell sorting for the combination ofmarkers specifically expressed on non-myocyte cells (in PE: CD31, CD90,CD140B, CD49A). (c) Flow cytometric analysis of the presort cells,PE-negative (LIN⁻) and PE-positive (LIN⁺) samples for SIRPA. (d)Quantification of non-myocyte markers in at day20 of differentiation (asshown in (a)), n=4. (e) Quantification of SIRPA-positive cells in PS,LIN− and LIN+ fractions after cell sorting. Asterisks indicatestatistical significance as determined by student's t-test, ***(p≤0.001), n=3. (f) QPCR analysis of the presort (PS), LIN⁻ and LIN⁺samples for non-cardiac markers (PECAM1, PDGFRB, THY1 and DDR2) andcardiac specific genes (SIRPA, NKX2-5, MYH6, and MYL7). Bars representmean±standard error of the mean. Asterisks indicate statisticalsignificance as determined by student's t-test, * (p≤0.05), ** (p≤0.01),*** (p≤0.001), n=3.

FIG. 8 shows differentiation kinetics of the NKX2.5-GFP HES3 hESC line.Flow cytometric analysis of EBs derived from the NKX2.5-GFP hESC line atvarious times during differentiation. GFP expression is first detectedat day8 of differentiation and increases over time with maximumexpression at day20.

FIG. 9 shows SIRPA expression kinetics of the NKX2.5-GFP HES3 and theHES2 hESC lines. (a) Analysis and quantification of SIRPA+/NKX2.5-GFP+cells by flow cytometric analysis. EBs derived from the NKX2.5-GFP hESCline were analyzed at various times during differentiation, n=5. (b)Analysis and quantification of SIRPA+ cells by flow cytometric analysis.EBs derived from the HES2 hESC line were analyzed at various timesduring differentiation, n=8. d0=undifferentiated ES cells,d5-d20=differentiated EBs at day5-day20.

FIG. 10 shows flow cytometry analysis strategy and staining controls.(a) Flow cytometric analysis of day20 EB-derived cells. All cells werestained with the viability dye DAPI and only DAPI-negative cells(=viable cells) were analyzed for each experiment. (b) Viable singlecells were further determined by FSC/SSC (cell size and granularity) inorder to exclude debris and doublets or cell clumps. (c) Unstainedcontrol of EB-derived cells at day20 of differentiation. (d) Flowcytometric analysis of day20 EB-derived cells with the SIRPA-PE-Cy7antibody and the corresponding IgG control. (e) Flow cytometric analysisof day20 EB-derived cells with the SIRPA-biotin/Streptavidin-APC(SIRPA-bio/SA-APC) antibody combination, the corresponding IgG controland secondary antibody only staining. (f) Comparison of cell sizebetween SIRPA− and SIRPA+ cell populations (from (e)) by FSC and SSC.

FIG. 11 shows Western Blot analysis and confirmation of the specificityof the SIRPA antibody. (a) Western Blot analysis of 3 samples from day20(d20) differentiation cultures compared to undifferentiated ES cells(d0). The SIRPA SE5A5 antibody was used and Ponceau staining is shownfor loading control. (b) Co-immunoprecipitation with the SIRPA SE5A5aantibody with controls. SIRPA runs at the predicted size, as previouslydescribed and analyzed in Timms et al., 1999.

FIG. 12 shows a comparison of SIRPA antibody staining with mito trackerdye retention labeling. (a) Flow cytometric analysis of mito tracker dyelabelling at day 5, 8, 12 and 20 of differentiation from HES2 hESCs. (b)Flow cytometric analysis of SIRPA at day 5, 8, 12 and 20 ofdifferentiation from HES2 hESCs. (c) Co-staining of SIRPA and mitotracker dye labelling followed by flow cytometric analysis at day 5, 8,12 and 20 of differentiation from HES2 hESCs.

FIG. 13 shows co-expression of SIRPA and cTNT. Cells were stained forSIRPA first, then fixed (4 % PFA, 20 min), followed by intracellularstaining for cTNT. Since both primary antibodies have been raised inmouse, appropriate controls are shown as well. Cells were stained foranti-SIRPA-biotin/Streptavidin-APC (SIRPA single stain),anti-SIRPA-biotin/Streptavidin-APC and anti-mouse-PE (control todemonstrate that the secondary antibody for cTNT does not recognizeSIRPA after fixation), anti-SIRPA-biotin/Streptavidin-APC and anti-cTNTand anti-mouse-PE (SIRPA and cTNT co-staining).

FIG. 14 shows analysis of Sirpa expression in mouse embryonic stemcell-derived cardiomyocytes and adult mouse tissue samples. (a) Flowcytometric analysis of mESC-derived cardiac EB cultures. Cells stainedfor Sirpa-AP, fixed with 4% PFA and stained with cTnT/antimouse-PE.Sirpa-expressing cells did not co-stain with cTnT-expressing cells,suggesting that cardiomyocytes derived from mES cells do not expressSirpa. (b) Flow cytometric analysis of mESC-derived cardiac EB cultures.Sirpa-positive cells co-stain with CD45-PE-Cy7, suggesting that theSirpa-positive cells present in these cultures represent hematopoieticcells, which have previously been described to express Sirpa (ref). (c)QPCR analysis of Sirpa in adult mouse tissue samples. TA, tibialisanterior muscle; GA, gastrocnemius muscle; GI, gastrointestinal tract;RT, reverse transcriptase control; ESCM, mouse embryonic stem cellderived cardiomyocytes day7 of differentiation (Kattman et al., 2011).Mouse brain tissue was used as positive control. (d) Western blotanalysis of adult heart, brain and kidney tissue from control (c) andSirpa-deficient mice (ko) (Timms et al., 1999) and mouse ESC-derivedcardiomyocytes (d). Sirpa expression was solely detected in the braintissue of control mice, but not in any of the Sirpa-deficient samples orin the control heart, kidney or mESC-derived samples. Antibodies #16 and#9 (specific for cytoplasmic domain, common to all Sirpa isoforms,AB#16, AB#9) were used as described in Timms et al., 1999. ABCAM:anti-Sirpa antibodies (Abcam, 8120).

FIG. 15 shows analysis of purity of SIRPA− and SIRPA+ fractions afterFACS. (a) Flow cytometric analysis of presort, SIRPA− and SIRPA+fraction for SIRPA after cell sorting. (b) quantification of SIRPA+cells in presort, SIRPA− and SIRPA+ fraction after cell sorting, n=3.

FIG. 16 shows enrichment of cardiomyocytes from hESC-derived cultures bycell sorting based on SIRPA expression. (a) Flow cytometric analysis ofSIRPA expression at day (d)8, d12 and d20 of differentiation fromNKX2.5-GFP HES3 hESCs. Fluorescent-activated cell sorting (FACS) forSIRPA was performed at d8, d12 and d20 and the presort (PS), SIRPA+ andSIRPA fractions were analysed for cardiac TroponinT (cTnT) expression byintracellular flow cytometry. The frequency of cTnT+ cells at d8, d12and d20 was significantly higher in the SIRPA+ fraction (day8:89.8%±1.9, day12: 95.0±1.3, day20: 89.4±4.4), compared to SIRPA− cells(day8: 9.9±1.7, day12: 21.9±2.5, day20: 5.2±0.5), n=3. (b) QPCR analysisof PS, SIRPA+ and SIRPA− cells after cell sorting. Expression of markersspecific for the cardiac lineage (NKX2.5, MYH6, MYH7 and MYL7) wassignificantly higher in the SIRPA+ compared to SIRPA− fraction at allstages analyzed (d8, d12 and d20). Expression of markers for thenon-cardiac lineages (PECAM and DDR2) segregated to the SIRPA− fractionand the PS cells, n=3.

FIG. 17 shows isolation of SIRPA+ cardiomyocytes via bead sorting. (a)Flow cytometric analysis of SIRPA. HES2-derived EBs were sorted usingthe Miltenyi magnetic bead sorting system and PS, SIRPA+ and SIRPAfractions after sorting were analyzed for SIRPA expression. (b)intracellular cTnT flow cytometric analysis of PS, SIRPA+ and SIRPA−fractions.

FIG. 18 shows gene expression analysis of human adult tissue. (a) QPCRRT analysis of SIRPA. (b) QPCR RT analysis of CD47.

FIG. 19 shows expression of non-myocyte markers in Y2-1-deriveddifferentiation cultures. (a) Flow cytometric analysis of mattersspecifically expressed on non-myocyte (SIRPA−) cells in day 20differentiation cultures. (b) Quantification of expression ofnon-myocyte matters at day 20 of differentiation from Y2-1 iPS cells.

FIG. 20 is a table showing the efficiency of fluorescent-activated cellsorting (FACS) with the SIRPA antibody. (a) Recovery of SIRPA− cellsafter FACS of EB-derived cells from HES2 at day20 of differentiation,n=8. (b) Recovery of SIRPA+ cells after FACS of EB-derived cells fromHES2 at day20 of differentiation, n=9. Total cell #=total cells passedthrough the flow cytometer; SIRPA− (SIRPA+)#=total SIRPA−(SIRPA+) cellsrecovered after the sorting procedure; SIRPA−(SIRPA+)% =percentage ofSIRPA−(SIRPA+) cells determined by staining with the SIRPA antibody;SIRPA−(SIRPA+) exp cell #=cells number of SIRPA−(SIRPA+) cells expectedbased on staining with the SIRPA antibody and on total cell numbersorted; Eff SIRPA−(SIRPA+)=efficiency of SIRPA−(SIRPA+) cell recovery:SIRPA−(SIRPA+) cell #/SIRPA−(SIRPA+) exp cell #; EffSIRPA−(SIRPA+)=efficiency of SIRPA−(SIRPA+) cell recovery in percentage.

FIG. 21 is a table showing the efficiency of fluorescent-activated cellsorting (FACS) with the nonmyocyte markers. (a) Recovery of LIN− cellsafter FACS of EB-derived cells from HES2 at day20 of differentiation,n=6. (b) Recovery of LIN+ cells after FACS of EB-derived cells from HES2at day20 of differentiation, n=6. Total cell #=total cells passedthrough the flow cytometer; LIN−(LIN+)#=total LIN−(LIN+) cells recoveredafter the sorting procedure; LIN−(LIN+)% =percentage of LIN−(LIN+) cellsdetermined by staining with the LIN antibodies; LIN−(LIN+) exp cell#=cells number of LIN−(LIN+) cells expected based on staining with theLIN antibodies and on total cell number sorted; EffLIN−(LIN+)=efficiency of LIN−(LIN+) cell recovery: LIN−(LIN+) cell#/LIN−(LIN+) exp cell#; Eff LIN−(LIN+)=efficiency of LIN−(LIN+) cellrecovery in percentage.

DETAILED DESCRIPTION

There is described herein the use of a high throughput flow cytometryscreen to identify cell surface markers specific for humancardiomyocytes. Here we report that the cell surface receptor SIRPA isexpressed on hPSC-derived cardiomyocytes as well as on human fetalcardiomyocytes. Using cell sorting with an antibody against SIRPA wedemonstrate that it is possible to isolate populations consisting of upto 98% cardiomyocytes from hPSC differentiation cultures.

Cell surface antigen, SIRPA (also known as CD172a, BIT, SHPS1), can befound specifically and exclusively on cardiac progenitor cells and ontroponin T-positive cardiomyocyte cells generated from human pluripotentstem cells (hPSCs) under appropriate differentiation conditions.

Prior to the present application, there was no indication or evidence inthe art that SIRPA is expressed on developing mouse or humancardiovascular cells. RNA expression of human SIRPA has been found indifferent parts of the brain as well as in blood and at low levels inthe lung. However, SIRPA RNA expression has not been found in the heart(http://biogps.gnf.org). SIRPA protein expression has been detected inthe brain, in blood and lymphoid tissues and in the colon, and atmoderate to weak levels in placenta, pancreas, spleen, bladder andstomach (http://www.proteinatlas.org/). However, no protein expressionhas been reported for the adult human heart. As such, the discovery thatSIRPA is expressed in hPSC-derived cardiac progenitor cells andcardiomyocyte cells is both novel and surprising.

In one example, the use of a SIRPA binding moiety, such as a SIRPAantibody, provides a simple and novel method to identify, monitor andisolate cardiomyocyte cells and their progenitor cells from populationsderived from human embryonic stem cells and induced pluripotent stemcells. Cell isolation is easy and efficient, yielding populations. Inone embodiment, consisting of greater than 90% cardiomyocyte cells thatremain viable and can be used for the applications disclosed herein.

SIRPA was identified as a potential cardiac marker in a screen of over350 commercially available antibodies supplied by the Ontario Institutefor Cancer Research Antibody Core Facility. The antibodies were screenedagainst hESC-derived populations representing different stages ofcardiac development generated by the directed differentiation of thehESCs using a previously published protocol (Yang et al., 2008).³Antibodies that stained cell populations of similar size to thecardiomyocyte population in the differentiation cultures (as defined bycardiac troponin T (cTnT) staining) were investigated further and usedfor cell sorting. Of the 350 surface antibodies, one antibody, SIRPA,specifically and exclusively stained the hESC-derived cardiomyocytepopulation.

Cells isolated based on SIRPA expression represent a novel source ofhighly enriched pluripotent stem cell-derived cardiomyocyte progenitorcells (e.g. at the onset of Nkx2.5 expression but before cellcontraction and expression of the cardiac-specific structural proteins)and cardiomyocyte cells for various applications, including but notlimited to the establishment of patient-specific disease models as wellas genetic, epigenetic and proteomic analyses of cardiac progenitorcells and cardiomyocyte cells from normal and patient-specificpluripotent stem cells.

The specific expression of SIRPA on cardiac cells and their precursorssuggests a function for this receptor and its downstream signallingpathways during cardiac development and differentiation.

SIRPA can also be used as a negative marker for cell sorting experimentsto enrich for non-cardiogenic PSC-derived lineages such as includingthose derived from the somite (progenitor cells of skeletal muscle,bone, and cartilage/chondrocytes).

Therefore, in one aspect, there is provided a method of enriching apopulation of cells for cardiomyocyte cells and cardiomyocyte progenitorcells comprising providing the population of cells from whichcardiomyocyte cells and cardiomyocyte progenitor cells are to beisolated; and isolating from the population, cells expressing SIRPA;wherein the population of cells comprises a population of humanpluripotent stem cells induced to differentiate into cardiomyocyte cellsand cardiomyocyte progenitor cells.

In one embodiment, the human pluripotent stem cells are embryonic stemcells. In another embodiment, the human pluripotent stem cells areinduced pluripotent stem cells.

In some embodiments, the human pluripotent stem cells are exposed to anamount of at least one inducing agent effective to induce celldifferentiation.

In a preferable embodiment, the at least one inducing agent comprises acytokine. The at least one inducing agent may comprise activin A,preferably at a concentration of up to 40 ng/ml, further preferably at aconcentration of about 6 ng/ml or about 30 ng/ml, the at least oneinducing agent may also independently comprise bone morphogeneticprotein 4, preferably at a concentration of up to 40 ng/ml, furtherpreferably at a concentration of about 10 ng/ml.

In some embodiments, the human pluripotent stem cells are furtherexposed to a bone morphogenetic protein inhibitor, preferably selectedfrom the group consisting of Dorsomorphin, Noggin and soluble bonemorphogenetic protein receptors.

In some embodiments, the human pluripotent stem cells are furtherexposed to at least one of VEGF, DKK and bFGF

In some embodiments, the human pluripotent stem cells are exposed to theinducing agent for between about 1 and about 5 days, preferably about 3days.

In some embodiments, the time between the initiation of induction of thehuman pluripotent stem cells and isolating the cells expressing SIRPA isbetween about five days and about forty-five days, preferably betweenabout 8 and about 25 days.

In some embodiments, the cells expressing SIRPA are isolated after theonset of SIRPA expression by the cells, which appears around the time ofonset of Nkx2.5 expression by the cells. Preferably, the cells havingthe SIRPA cell surface antigen are isolated between the time of theonset of Nkx2.5 expression by the cells and the time of the onset ofcontraction and expression of the cardiac-specific structural proteinsby the cells.

In some embodiments, the method further comprises depleting from thepopulation, cells expressing at least one of CD90, CD31, CD140B andCD49A, preferably using a corresponding antibody.

Methods for isolating cells expressing a particular molecule, in thiscase SIRPA, are known to a person skilled in the art. In someembodiments, the presence of SIRPA is directly used to isolate cells byusing a SIRPA-specific ligand, preferably using an anti-SIRPA antibodyor antibody fragment, or antibody-like molecule, and further preferablyan anti-SIRPA antibody. In some embodiments, the cells are then isolatedusing magnetic beads and/or flow cytometry. Alternatively, cellsexpressing SIRPA may be indirectly selected. For example, in someembodiments, the cells in the population comprise a reporter geneoperably linked to regulatory control elements of the SIRPA locuswhereby the reporter gene is expressed in cells that express SIRPA andthe step of isolating the cells expressing SIRPA comprises isolatingcells expressing the reporter gene. In one preferable embodiment, thereporter gene confers resistance to a cytotoxic agent. In anotherpreferable embodiment, the reporter gene is a cell surface tag.

In some embodiments, the enriched population of cells comprises at least60%, preferably at least 90%, further preferably 98%, cardiomyocytecells and cardiomyocyte progenitor cells.

In a further aspect, there is provided an enriched population ofcardiomyocyte cells and cardiomyocyte progenitor cells obtained usingany one of the methods described herein.

In a further aspect, there is provided an isolated population of cellsenriched for cardiomyocyte cells and cardiomyocyte progenitor cells,wherein the population of cells comprises at least 60%, preferably atleast 90%, further preferably 98%, cardiomyocyte cells and cardiomyocyteprogenitor cells.

In a further aspect, there is provided the use of SIRPA for isolatingcardiomyocyte cells and cardiomyocyte progenitor cells from a populationof cells, wherein the population of cells compose a population of humanpluripotent stem cells induced to differentiate into cardiomyocyte cellsand cardiomyocyte progenitor cells.

In a further aspect, there is provided a method of depleting apopulation of cells for cardiomyocyte cells and cardiomyocyte progenitorcells comprising: providing the population of cells from whichcardiomyocyte cells and cardiomyocyte progenitor cells are to bedepleted; and depleting from the population, cells expressing SIRPA;wherein the population of cells comprises a population of humanpluripotent stem cells induced to differentiate into cardiomyocytecells, cardiomyocyte progenitor cells, and non-cardiomyocytes.

In a further aspect, there is provided a method of enriching apopulation of cells for cardiomyocyte cells and cardiomyocyte progenitorcells comprising: providing the population of cells from whichcardiomyocyte cells and cardiomyocyte progenitor cells are to beisolated; and depleting from the population, cells expressing at leastone of CD90, CD31, CD140B and CD49A: wherein the population of cellscomprise a population of human pluripotent stem cells induced todifferentiate into cardiomyocyte cells, and cardiomyocyte progenitorcells, and non-cardiomyocytes.

The term “enriching”, as used in the context of tee present invention,includes any isolation or sorting process that increases the relativeabundance of a desired cell type, or cell types, in a population ofcells.

As used herein, the term “cardiomyocyte cells” refers to the cells thatcomprise cardiac muscle.

The term “cardiomyocyte progenitor cells” means progenitor cells derivedfrom human pluripotent stem cells that have the capacity todifferentiate into cardiomyocyte cells.

As used herein, the process of “isolating cells” refers to any methodknown to those skilled in the art for sorting cells including, but notlimited to, flow cytometry, fluorescence activated cell sorting,magnetic separation using antibody-coated magnetic beads, affinitychromatography, and the exploitation of differences in physicalproperties (e.g., density gradient centrifugation).

“Embryonic stem cells” (“ESC”) are pluripotent stem cells that arederived from early-stage embryos.

“Induced pluripotent stem cells” (“iPSC”), as used in the context of thepresent invention, is a type of pluripotent stem cell that has beenartificially derived from a non-pluripotent cell by inducing theexpression of specific genes.

The term, “cell surface antigen”, refers to antigens on surfaces ofcells that are capable of being recognized by the immune system andbinding significantly to an antibody.

As used herein, the phrase “induced to differentiate” refers to anymethod known in the art used to initiate the differentiation of humanpluripotent stem cells into specialized cell types. These methods mayinclude exposure of the human pluripotent stem cells to an inducingagent.

As used herein, the term “inducing agent” refers to any agent capable ofinitiating differentiation of hPSCs into specialized cell types,including cardiomyocyte cells and cardiomyocyte progenitor cells.Inducing agent therefore includes cytokines, including but not limitedto activin A, bone morphogenetic protein 4 (BMP4), basic fibroblastgrowth factor (bFGF, also known as FGF2), vascular endothelial growthfactor (VEGF, also known as VEGFA), dickkopf homolog 1 (DKK1), andcombinations therefrom.

Methods for inducing human pluripotent stem cells to differentiate intocardiomyocyte cells and cardiomyocyte progenitor cells are known to aperson skilled in the art (for e.g., see Yang et al.³, and Laflamme etal.¹⁹). In some embodiments, induction conditions (e.g. concentrationsof the inducing agents and timing of their use) can be optimized bymeasuring SIRPA concentration in the resulting enriched population.

The ability to generate cells of the cardiac lineage from humanpluripotent stem cells hPSCs (including embryonic stem cells; hESCs andinduced pluripotent stem cells; hiPSCs) provides a novel and unlimitedsupply of human cardiomyocyte cells that will be useful for: 1)predictive drug toxicology and drug discovery, 2) transplantation forthe treatment of cardiovascular disease and 3) modeling cardiovasculardevelopment and disease in vitro.

The following examples are illustrative of various aspects of theinvention, and do not limit the broad aspects of the invention asdisclosed herein.

EXAMPLES Materials and Methods HPSC Maintenance and Differentiation

HPSCs were maintained as described²⁶. Embryoid bodies (EBs) weredifferentiated to the cardiovascular lineage as previouslydescribed^(2,3) (FIG. 1a ). In brief: EBs were generated on day0 (d0)and BMP4 (1 ng/ml) was added for the first day of differentiation(d0-d1). At d1, EBs were harvested and resuspended in induction medium(basic fibroblast growth factor (bFGF; 2.5 ng/ml), Activin A (6 ng/ml)and BMP-4 (10 ng/ml)). The medium was changed on d4 and was supplementedwith vascular endothelial growth factor (VEGF; 10 ng/ml) and DKK (150ng/ml). Media was changed again on d8 and was supplemented with VEGF (20ng/ml) and bFGF (10 ng/ml). EBs were cultured in StemPro-34 (Invitrogen)throughout the experiment. Cultures were maintained in a 5% CO₂, 5% O₂,90% N₂ environment from d0-d12 and were then transferred into a 5%CO₂/air environment for the remainder of the culture period.

NKX2-5-GFF hESCs were generated by targeting sequences encoding GFP tothe NKX2-5 locus of HES3 cells using previously described protocols²⁷(D.E., A.G.E. and E.G.S., manuscript submitted).

Work involving human tissue collection and analysis was carried out inaccordance with and approved through the Human Ethics Committee at theUniversity Health Network.

Flow Cytometry and Cell Sorting

Dissociation procedure for day5 to day12 EBs: EBs generated from hPSCdifferentiation experiments were dissociated with 0.25% trypsin/EDTA.Dissociation procedure for day13 and older EBs and human fetal tissue:EBs generated from hPSC differentiation cultures were incubated incollagenase type II (1 mg/ml; Worthington, LS004176) in Hanks solution(NaCl 136 mM, NaHCO3 4.16 mM, NaPO4 0.34 mM, KCl 5.36 mM, KH2PO4 0.44mM, Dextrose 5.55 mM, Hepes 5 mM) over night at room temperature withgentle shaking²⁵. The following day, the equivalent amount ofdissociation solution (in Hanks solution: taurin, 10 mM, EGTA 0.1 mM,BSA 1 mg/ml, collagenase type II 1 mg/ml) was added to the cellsuspension and the EBs wore pipetted gently for complete dissociation.Cells were centrifuged (1000 rpm, 5 min) and filtered. For EBs past day40 of differentiation, additional treatment with 0.25% trypsin/EDTA maybe required in order to obtain complete dissociation into single cells.

Cells were stained at a concentration of 2.5×10⁶ cells/ml withanti-KDR—allophycocyanin (R&D Systems; 1:10) andanti-PDGFRA—phycoerythrin (R&D Systems; 1:20),anti-SIRPA—IgG-phycoerythrin-Cy7 (clone SE5A5; BioLegends;1:500)^(10,29), anti-SIRPA—IgG-biotin (clone SE5A5; BioLegends;1:500)¹⁰, anti-cardiac isoform of Troponin T (cTNT) (clone 13-11;NeoMarkers; 1:400), goat anti-mouse IgG—allophycocyanin (BD; 1:200),Streptavidin—allophycocyanin (BD; 1:200), anti-IgG1κ—phycoerythrin-Cy7(clone MOPC-21; BioLegends; 1:500), anti-IgG1κ-biotin (clone MOPC-21;BioLegends; 1:500).

For cell surface markers, staining was carried out in PBS with 10% FCS.For intracellular proteins, staining was carried out on cells fixed with4% paraformaldehyde (Electron Microscopy Sciences, Hatfield, Pa., USA)in PBS and stainings were performed in PBS with 10% FCS and 0.5% saponin(Sigma). Stained cells were analyzed using an LSRII flow cytometar (BD).For fluorescent activated cell sorting, the cells were sorted at aconcentration of 10⁶ cells/ml in IMDM/6% FCS using a FACSAriaTMII (BD)cell sorter (SickKids-UHN Flow Cytometry Facility, Toronto, ON, Canada).In order to prevent cell death due to pressure and sheer stress, allsorts were performed with a 100 micron nozzle. For magnetic beadsorting, the Miltenyi MACS bead sorting system was used and theexperiments were carried out according to the manufacturer's guidelinesand the sorting conditions for dim markers. For the high throughput flowcytometry analysis the BD high throughput sampler (HTS) for the LSRIIwas used according to the manufacturers guidelines. Data were analysedusing FlowJo software (Treestar, Ashland, Oreg., USA).

Immunostaining

Immunostaining was performed as previously described¹³ using thefollowing primary antibodies: rabbit anti-cardiac Troponin I (Abcam;1:100), mouse anti-SIRPA (BioLegends; 1:100). Secondary antibodies usedwere: goat anti-mouse IgGCy3 (Jackson ImmunoResearch; 1:400), donkeyanti-mouse IgG-Alexa 488 (Invitrogen; 1:400). DAPI was used tocounterstain nuclei. Mito Tracker Red (Invitrogen) was used to stainmitochondria. The stained cells were visualized using a fluorescencemicroscope (Leica CTR6000) and images captured using the LeicaApplication Suite software.

Quantitiative Real-Time PCR

Total RNA was prepared with the RNAqueous-Micro Kit (Ambion) and treatedwith RNase-free DNase (Ambion). 500 ng to 1 μg of RNA was reversetranscribed into cDNA using random hexamers and Oligo (dT) withSuperscript III Reverse Transcriptase (Invitrogen). QPCR was performedon a MasterCycler EP RealPlex (Eppendorf) using QuantiFast SYBR GreenPCR Kit (Qiagen) as described previously¹³. Expression levels werenormalized to the housekeeping gene TATA box binding protein (TBP). Inaddition to TBP for normalization across samples, genomic DNA was usedas a DNA standard. The copy number of the target gene present in thegenomic DNA can be directly calculated (Human genome size: 2.7×10⁹ bp(=1.78×10¹² Daltons), corresponds to 6.022×10²³ copies of a single copygene; 1 μg of genomic DNA corresponds to 3.4×10⁵ copies of a single copygene). The Y-axis of RT-qPCR graphs represents copy numbers of the geneof interest divided by copy numbers of TBP, and therefore represents anarbitrary but absolute unit, that can be compared between experiments.

Total human adult heart RNA was purchased from Ambion and a total humanRNA master panel was purchased from Clontech.

Results & Discussion

Identification of Novel Markers Expressed on hESC-Derived Cardiomyocytes

When induced with appropriate concentrations of Activin A and BMP4 (FIG.1a ), the HES2 hESC line efficiently and reproducibly differentiates togenerate cardiovascular lineage cells^(2,3). Kinetic analyses of thedifferentiation cultures revealed a step-wise developmental progressionfrom a primitive streak-like population defined by BRACHYURY (T)expression (days 2-4) to the development of the early mesoderm (MESP1;days 3-4) and the emergence of NKX2-5 and ISLET1 (ISL1) positive cardiacprecursors (days 4-8). Contracting cardiomyocytes were first detectedbetween days 9 and 12 of differentiation, coincident with theup-regulation of MYH6 (αMHC), MYH7 (βMHC) and MYL7 (MLC2a) and laterMYL2 (MLC2v) expression (FIG. 1b ). The levels of expression of some ofthe cardiac specific genes in the hESC-derived populations wereconsiderably lower than the levels found in fetal and adult hearttissue. Low levels of NEUROD1 and FOXA2 expression indicate that thecultures were not contaminated with substantial numbers of neuroectodermor endoderm-derived cells. To be able to monitor cardiomyocytedevelopment in real time, we applied the above protocol to an NKX2-5-GFPreporter hESC line that contains the EGFP cDNA inserted into the NKX2-5locus of HES3 hESCs (Elliott et al., manuscript submitted). The firstNKX2-5-GFP⁺ cells developed between days 7 and 8 of differentiation. Thesize of the NKX2-5-GFP⁺ population increased with time, reaching amaximum between days 12-20 (FIG. 8). Analysis of NKX2-5-GFP ESC-derivedembryoid bodies (EBs) under epifluorescence confirmed nuclear GFPexpression in the majority of the cells. The kinetics of NKX2-5-GFPexpression closely parallels the onset of NKX2-5 expression in the HES2cultures, indicating that cardiac specification from both hESC linestakes place between days 6 and 8 of differentiation (FIG. 1b , FIG. 8).The high proportion of NKX2-5-GFP⁺ cells in day 20 cultures demonstratesthat the differentiation protocol used efficiently promotes thegeneration of cardiomyocytes from this hESC line.

To determine if the above developmental stages can be distinguished bycell surface markers, we carried out a screen of 370 known antibodies(http://data.microarrays.ca/AntibodyWeb) using day 8, 12, and 20populations generated from the GFP-NKX2-5 cell line. The initial screenfocused on identifying antibodies that recognized antigens present onthe NKX2-5-GFP⁺ population. From this screen, we identifiedsignal-regulatory protein alpha (SIRPA, also known as SHPS-1, SIRPA) asa potential cardiac-specific marker, as the anti-SIRPA antibody¹⁰stained the majority of the NKX2-5-GFP⁺ cells and almost none of thenegative cells (FIG. 2a ). From the panel of antibodies analyzed, SIRPAwas the only one that displayed this cardiomyocyte specific expressionpattern. SIRPA was first detected on the emerging GFP-NKX2-5⁺ cells atday 8 of differentiation, a population considered to represent thecardiac precursor stage of development. Expression was maintained onGFP-NKX2-5⁺ population throughout the 20-day time course of theexperiment (FIG. 2a , FIG. 9a ). No SIRPA⁺ cells were detected inundifferentiated hESC populations or in the day 5 cardiac mesodermpopulation characterized by co-expression of KDR and PDGFRA (FIG. 2a anddata not shown)². Analyses of EBs generated from the non-geneticallymodified HES2 line revealed a similar staining pattern with theanti-SIRPA antibody. SIRPA⁺ cells were first detected at days 7-8 ofdifferentiation and the percentage of positive cells increasedsignificantly over the next 2-4 days (FIG. 2b , FIG. 9b ). Both thedirectly conjugated (SIRPA-PE-CY7) and the botinylated (SIRPA-bio)antibodies stained similar portions of the day 20 EB population (FIG.10a-e ). Interestingly, the SIRPA⁺ cells detected in day 20 EBs appearto be substantially larger than those found in the SIRPA⁻ population(FIG. 10f ), suggesting that cell size of these populations can beassessed by flow cytometry. To confirm the specificity of the SIRPAantibody, we carried out Western Blot analyses and immunoprecipitationfollowed by Western Blot analysis (FIG. 11). These experimentsdemonstrated the presence of SIRPA protein in 3 independent day 20EB-derived populations, but not in undifferentiated hESCs (FIG. 11a ).Immunoprecipitation analyses revealed a band the size of that previouslydescribed for the SIRPA protein (FIG. 11b )¹¹.

Co-staining of SIRPA and cTNT by flow cytometry displayed clearco-expression of the two markers (FIG. 12 a/b). Indicating that SIRPAwas specifically expressed on the cardiomyocyte lineage indifferentiated populations generated from the non modified HES2 cellline.

RT-qPCR analyses revealed an expression pattern for SIRPA that closelymirrored the flow cytometry antibody staining profile, with anup-regulation of SIRPA mRNA between days 6 and 8 of differentiation,followed by persistence of expression over the 42-day time course.Expression of CD47, the ligand for SIRPA, paralleled that observed forSIRPA (FIG. 2c ). Flow cytometric analysis of CD47 reflected the geneexpression pattern, showing low levels of staining on undifferentiatedES cells and on day 5 differentiation cultures, followed by broadstaining on the entire population at days 8 and 20 (data not shown).

Immunofluorescence analysis of monolayer cultures derived from day 20EBs revealed SIRPA surface expression exclusively on cardiomyocytes, ascharacterized by co-expression with cardiac TroponinI (cTNI) (FIG. 2d )The respective controls (IgG and secondary antibody only) did not showany signal (data not shown). Collectively, these kinetic studies showthat expression of SIRPA uniquely marks the cardiac lineage in hESCdifferentiation cultures, beginning with the emergence of NKX2-5⁺precursor cells and persisting through the development and expansion ofcontracting populations.

Haitori et al recently demonstrated it was possible to isolatecardiomyocytes based on mitochondria content, as measured by retentionof a mito tracker dye⁹. Comparison of mito tracker dye labeling withSIRPA staining indicated that both procedures mark the samecardiomyocyte population in day 20 EBs (FIG. 13c ). The dye retentionapproach was, however, less useful in tracking the onset ofcardiovascular development, as it marked a less distinct population atday 12 of differentiation and almost no cells at day 8 (FIG. 13 a/b). Incontrast, a substantial SIRP⁺ population could be clearly resolved atboth these time points indicating that this surface marker allows one tomonitor and isolate cells from different stages of cardiac development,whereas labeling with the mito tracker dye can only be used onpopulations containing relatively mature cardiomyocytes.

In contrast to the human cells, Sirpa was not detected on mouseESC-derived cardiomyocytes by antibody staining (FIG. 14a ). Sirpa⁺populations in the culture were cardiac Troponin T (cTnT) negative andCD45 positive, indicating that they represent hematopoietic cells (FIG.14 a/b). Gene expression analyses confirmed the flow cytometric data,and showed only low levels of Sirpa mRNA in the mESC-derivedcardiomyocytes as well as in adult mouse atrial and ventricular tissues,compared to high expression in the brain (FIG. 14c ). Expression of theonly other known Sirp family member in the mouse, Sirpb, could not bedetected in any of these tissues by qPCR (data not shown). Western blotanalysis of control and Sirpa-deficient mouse tissue confirmed highSirpa expression in the brain of control mice, but not in any of thetissues derived from Sirpa-deficient mice (FIG. 14d ). Most importantly,no Sirpa expression was detected in the heart, kidney or mESC-derivedcardiomyocytes from control mice.

Differences in SIRPA function and protein homology for mouse and humanhave been described previously for the interaction of macrophages andred blood cells¹².

Purification of Cardiomyocytes From hESC-Derived Populations

To assess whether expression of the SIRPA surface receptor can be usedto generate enriched populations of cardiomyocytes, SIRPA-positive(SIRPA⁺) and SIRPA-negative (SIRPA⁻) fractions were isolated by cellsorting from HES2-derived EBs at days 8, 12 and 20 of differentiationand analyzed for expression of cardiac troponin T (cTNT) byintracellular flow cytometry (FIG. 3a ). Analyses of the presort(unsorted, PS) populations demonstrated that cTNT expression closelyparalleled that of SIRPA at the corresponding stages duringdifferentiation (PS: d8, d12, d20). Following sorting, the SIRPA⁺fractions from each stage were highly enriched for cTNT⁺ cardiomyocytes,whereas the SIRPA⁻ fractions were depicted of these cells. It is unclearif the low numbers of cTNT⁺ cells present in the SIRPA⁻ fractions arecontaminants from the sorting procedure or represent true SIRPA-negativecardiomyocytes. FACS based separation in multiple experimentsreproducibly yielded significantly enriched populations ofcardiomyocytes (SIRPA⁺: day8 (96.2%±1.9), day12 (94.4%±1.7), day20(89.6%±3.6); SIRP⁻: day8 (13.0%±2.1), day12 (14.3%±3.9), day20(15.7%±6.0)) (FIG. 3b ). The purity of the SIRPA⁺ 0 and SIRPA⁻ sortedpopulations and the efficiency of cell recovery from the sortingprocedure is summarized in FIG. 15 and FIG. 20 (Table 1).

Molecular analyses revealed that the SIRPA⁺ cells expressedsignificantly higher levels of NKX2-5, MYH6, MYH7 and MYL7 than theSIRPA⁻ population (FIG. 3c ), further demonstrating enrichment ofcardiomyocytes. As expected, SIRPA expression segregated to the SIRPA⁺population in contrast to the cardiac markers, non-myocyte markers suchas the fibroblast markers DDR2 and THY1 (CD90, data not shown) and theendothelial marker PECAM (CD31) were expressed at higher levels in theSIRPA⁻ population (FIG. 3c ).

When plated in monolayer cultures, cells from both SIRPA⁻ and SIRPA⁺fractions formed viable populations that could easily be maintained forseveral weeks. Contracting cells were detected in unsorted (PS) andSIRPA⁺-derived populations, but not in the population generated from theSIRPA⁻ cells. Immunohistochemical analysis revealed broad cTNIexpression in the SIRPA⁺ population confirming the high proportion ofcardiomyocytes in these cultures. Only few cTNI-positive cells weredetected in the SIRPA⁻ population (FIG. 3d )

As anticipated from the co-expression of SIRPA and NKX2-5-GFP, it wasalso possible to isolate populations enriched for cardiac lineage cellsfront NKX2-5-GFP HES3-derived cultures by sorting with the anti-SIRPAantibody. Cardiac precursors (day 8) and cardiomyocytes (days 12 and 20)defined by gene expression and cTNT staining, segregated to the SIRPA⁺fraction whereas non-myocyte cells were enriched in the SIRPA⁻population (FIG. 16).

To enable rapid processing of large numbers of cells, we also attemptedto isolate SIRPA cells by magnetic bead sorting. Isolation of SIRPA⁺cells from NKX2-5-GFP differentiation cultures by this approach resultedin populations highly enriched for cardiomyocytes similar to thosederived from FACS experiments (FIG. 17a-c ). However, with currentmagnetic bead sorting protocols a substantial amount of cells is lostduring the process, resulting in a lower efficiency of this approachcompared to FACS (compare FIG. 17d to FIG. 20 (Table 1)).

Taken together, the findings from these cell sorting studies dearlydemonstrate that SIRPA expression marks the cardiac lineage inhESC-derived differentiation cultures and that cell sorting with theanti-SIRPA antibody allows for the isolation of populations highlyenriched for cardiomyocytes.

Purification of Cardiomyocytes From Human Induced Pluripotent Stem Cells

To determine if SIRPA expression marked the cardiac lineage in otherhPSC-derived populations, we next analyzed EBs generated from twodifferent hiPSC lines, MSC-iPS1 (also known as Y2-1) and 38-2^(13,14).The efficiency of cardiac differentiation from both lines was low, asdemonstrated by the proportion of cTNT⁺ cells (MSC-iPS1: 12.2%±5.6,38-2: 26.7%±5.7; FIG. 4a ). Similar low levels of SIRPA expression weredetected in both EB populations. FACS of the SIRPA⁺ cells from both iPSClines yielded populations significantly enriched for cTNT⁺cardiomyocytes (SIRPA⁺: MSC-iPS1 (67.0%±3.6), 38-2 (71.4%±3.8); SIRPA⁻:MSC-iPS1 (4.9%±2.1), 38-2 (6.2%±0.9)) (FIG. 4a,b ). These SIRPA⁺populations expressed significantly higher levels of NKX2-5, MYH6, MYH7,MYL2 and MYL7 than the corresponding SIRPA⁻ cells. As observed with thehESC-derived cells, non-myocyte markers including DDR2, PDGFRB, THY1 andNEUROD segregated to the SIRPA⁻ fraction (FIG. 4b,c and data not shown).

Those data clearly document the utility of this marker for generatingenriched cardiac populations from a range of pluripotent stem celllines, including these that do not differentiate efficiently to thecardiac lineage with the current protocols.

SIRPA Expression in Human Fetal and Adult Heart Cells

To determine if SIRPA is expressed on primary human cardiomyocytes, wenext analyzed expression patterns in fetal (18-20 weeks of gestation)and adult heart tissue by RT-qPCR. As shown in FIG. 5 a, SIRPAtranscripts were detected in all fetal-derived heart tissue (left (LA)and right atrial (RA) cells, left (LV) and right ventricle (RV) cells,apex (AP) and atrioventricular junction (AVJ)), with comparable levelsto those found in day 20 hESC-derived cells (FIG. 3a ). SIRPA was notexpressed in undifferentiated hESCs (d0) or in control HEK (humanembryonic kidney) cells. Similar to the fetal heart, SIRPA expressionwas also detected in the adult heart, suggesting that its expressionmarks cardiomyocytes at different stages of human cardiac development.High levels of SIRPA were detected in the adult human brain and lung(FIG. 18a ) with low levels found in many other tissues. These lowlevels may reflect the presence of tissue macrophages that are known toexpress this receptor^(15,16). CD47, the SIRPA ligand was expressed inmost tissues, confirming the pattern described in previous studies (FIG.18b )¹⁵. Immunofluorescence staining showed that SIRPA was localized onthe surface membrane of the fetal ventricular cells but was not presenton other membrane fractions such as the mitochondrial membrane, asindicated by the lack of co-staining with Mito Tracker Red (FIG. 5b ).Flow cytometric analyses revealed a high proportion of SIRPA⁺ cells inall fetal heart tissues at levels that correlated with the percentage ofcTNT⁺ cells in the respective fractions (FIG. 5c, d ).

These findings demonstrate clearly that SIRPA is expressed on fatalcardiomyocytes as well as in adult heart, illustrating that itscardiac-specific expression is not an artifact of pluripotent stemcell-derived cultures.

Using SIRPA Expression to Monitor the Efficiency of hPSC Differentiation

Recently, we reported that co-expression of KDR and PDGFRA provides areliable method to monitor cardiac mesoderm induction followingtreatment with BMP4 and Activin A² (FIG. 6a ). While this study showedthat the induction of a KDR⁺PDGFRA⁺ population was an essential firststep in the generation of the cardiomyocyte population, not allKDR⁺PDGFRA⁺ populations differentiated to give rise to cardiac lineagecells (example of this type of population: induced with 30 ng/ml BMP4and no exogenous Activin A (A0)). To determine if SIRPA would moreaccurately predict cardiac potential of differentiating populations atan early stage, we monitored its expression in day 9 EBs induced withdifferent concentrations of Activin A and BMP4 (FIG. 6b ). The samepopulations were evaluated at day 5 for expression of KDR and PDGFRA(FIG. 6a ) and at day 20 for expression of cTNT (FIG. 6c ). While therewas little correlation between the size of the KDR⁺PDGFRA⁺ population atday 5 and the proportion of cTNT⁺ cells at day 20, the cultures with thelargest SIRPA population at day 9 (Activin A 6 ng/ml, BMP4 10 ng/ml)contained the highest number of cTNT⁺ cells at the later time point.SIRPA expression correlated well with cTNT output for most conditionstested and the highest levels of SIRPA predicted the highestcardiomyocyte development at day 20 (FIG. 6d ). These data demonstratethat expression of SIRPA at day 9 is a reliable indicator ofcardiomyocyte potential, and as such can be used to monitor and optimizeinduction protocols for directed differentiation of hPSCs to the cardiaclineage.

Enrichment of hPSC-Derived Cardiomyocytes Through Depletion of theNon-Myocyte Lineage Cells

In addition to antibodies that recognize cardiomyocytes, our nowcytometric screen also identified a panel of antibodies that marked thenon-myocyte population in the differentiation cultures. This set ofantibodies, including anti-CD90 (THY1, expressed on fibroblast cells),anti-CD31 (PECAM1, expressed on endothelial cells), anti-CD140B (PDGFRB,expressed on smooth muscle cells) and anti-CD49A (INTEGRIN1A), allrecognized different proportions of the SIRPA⁻ population of day 20HES2-derived EBs (FIG. 7 a/d). The combination of these antibodiesmarked the majority of non-myocyte (SIRPA⁻) cells in the culture (FIG. 7c. presort). To determine if it was possible to enrich forcardiomyocytes by depleting cells expressing the non-myocyte markers, wecombined these antibodies and sorted day 20 EBs into lineage-positive(LIN⁺) and lineage-negative (LIN⁻) fractions (FIG. 7b ). This approachhas the advantage in generating enriched populations free of any boundantibody or magnetic beads. As expected, the LIN⁻ population wassignificantly enriched for SIRPA⁺ cells, whereas the LIN⁺ population wasdepleted for the cardiomyocytes (FIG. 7 c/e). The efficiency of cellrecovery after FACS for LIN⁻ and LIN⁺ cells is summarized in FIG. 21(Table 2). Gene expression analyses revealed that non-myocyte specificgenes including PECAM1, PDGFRB, THY1 and DDR2 were primarily expressedin the LIN⁺ fraction, whereas cardiac gene expression was restricted tothe LIN⁻ fraction (FIG. 7f ). When plated on gelatin coated dishes orre-aggregated as cell clusters, the LIN⁻ fraction generated populationsthat contained a high proportion of contracting cardiomyocytes (data notshown). The same lineage cocktail of antibodies also marked thenon-myocyte (SIRPA−) fraction of the iPSC (MSC-iPS1)-derived day 20 EBpopulation (FIG. 19), indicating that this depletion approach can beapplied to different PSC lines with variable differentiationefficiencies.

Taken together, these data illustrate that cardiomyocytes can beenriched from hPSC-derived differentiation cultures by depletion of thenon-myocyte lineages. This method therefore represents an alternativeapproach to obtaining highly purified cardiomyocyte cultures and may assuch be used for strategies that require purified cardiomyocytepopulations free of any bound antibodies.

Advances in our understanding of the signaling pathways that regulatelineage specification has led to strategies for the efficient andreproducible directed differentiation of hPSCs to specific cell types¹.With respect to cardiac lineage development, protocols have beenestablished that promote the generation of mixed cardiovascularpopulations representing the major cell types found in the human heartincluding cardiomyocytes, endothelial cells, vascular smooth musclecells and fibroblasts. Cardiomyocytes typically represent between 10%and 70% of such mixed populations^(2,3), depending on the PSC line used.While such mixed populations have been used to demonstrate the potentialutility of the PSC-derived cells for predictive toxicology⁵, modelinghuman disease in vitro^(17,18) and transplantation based therapy forheart disease¹⁹, highly enriched and well defined cell populations willultimately be required to translate this potential into practicalapplications.

Our identification of SIRPA as a cardiomyocyte-specific marker nowenables, for the first time, easy and routine access to highly enrichedpopulations of cardiomyocytes from hESCs and hiPSCs. Those cardiomyocyteenriched populations can be isolated by FACS or magnetic bead sorting,the latter approach enabling the isolation of large numbers of cellsrequired for in vivo studies. Access to highly enriched populations ofcardiomyocytes through simple sorting approaches will enable thedevelopment of defined high throughput drug discovery and toxicologyassays, the detailed phenotypic evaluation of cells generated frompatient specific hPSCs, and the generation of defined populations safefor transplantation. The fact that SIRPA is expressed on cardiac lineagecells from the earliest cardiac stage to contracting and more maturecardiomyocytes will allow for comparisons of the in vivo potential ofthe different populations.

In addition to SIRPA, our screen also identified a panel of markersdefining the non-myocyte fractions of the PSC-derived cardiovascularpopulation. The markers used suggest that they represent a combinationof fibroblasts (CD90, THY1)²⁰, vascular smooth muscle cells (CD140B,PDGFRB)²¹ and endothelial cells (CD31, PECAM1). Access to enrichedpopulations of each of these cell types together with cardiomyocyteswill allow. Many of the proposed applications for PSC-derivedcardiomyocytes may require three-dimensional engineered tissue to moreaccurately reflect drug responses and function in the adult heart.Recent studies suggest that appropriate combinations of cardiac cells,endothelial cells and fibroblasts need to be incorporated into suchtissue constructs in order for them to function best in vitro or invivo²²⁻²⁴. Our ability to generate pure myocyte and non-myocytepopulations will allow for the generation of engineered constructsconsisting of varying proportions of different cell types, enabling usto determine the optimal proportion of each required to form hearttissue with structural and functional properties most similar to that ofthe human heart.

The specific expression pattern of SIRPA in the PSC-derived populationsand in the fetal heart tissue suggests that this receptor plays somefunctional role in the human cardiomyocyte lineage, perhaps as early asthe precursor stage of development. The fact that expression of theligand, CD47, is upregulated in parallel with SIRPA in the EBs and thatCD47 is found on a large proportion of the cells in the culture furthersupports the interpretation that this ligand/receptor pair plays a rolein the human cardiomyocyte development and/or function. One thoroughlystudied role for SIRPA is on macrophages, where it appears to mediate asignal to eliminate cells from the body that do not express the ligandCD47¹⁶. The only other suggested function in human cells is in thesmooth muscle lineage, where SIRPA has been shown to play an importantrole in mediating IGF-1-induced mitogenic signaling²⁶. Given that SIRPAwas not detected in mouse cardiomyocytes, it is possible that itsfunction in human cells may relate to aspects of cardiomyocytephysiology and/or function that differ between the two species.

In summary, the findings reported here demonstrate that expression ofSIRPA uniquely marks the cardiomyocyte lineage in PSC-differentiationcultures. Isolation of SIRPA⁺ cells by FACS or magnetic bead sortingprovides a simple approach for generating highly enriched populations ofcardiomyocytes from a broad range of PSC lines, including those that donot differentiate efficiently to the cardiovascular lineage usingcurrent protocols.

Although preferred embodiments of the invention have been describedherein, it will be understood by those skilled in the art thatvariations may be made thereto without departing from the spirit of theinvention or the scope of the appended claims. All references in thisdescription, including those in the following reference list, are herebyincorporated by reference.

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1-30. (canceled)
 31. A composition comprising: a cell populationcomprising one or more of human cardiomyocytes and human cardiomyocyteprogenitor cells; and a signal-regulatory protein alpha (SIRPA)-specificbinding member.
 32. The composition of claim 31, wherein the cellpopulation is derived from human pluripotent stem cells.
 33. Thecomposition of claim 32, wherein the human pluripotent stem cells areselected from embryonic stem cells (hESCs) or human induced pluripotentstem cells (iPSCs).
 34. The composition of claim 31, wherein theSIRPA-specific binding member is selected from the group consisting ofan anti-SIRPA antibody, an anti-SIRPA antibody fragment, and ananti-SIRPA antibody-like molecule.
 35. The composition of claim 31,wherein the cell population comprises at least 60% cardiomyocyte cells,cardiomyocyte progenitor cells, or both.
 36. The composition of claim35, wherein the cell population comprises at least 90% cardiomyocytecells, cardiomyocyte progenitor cells, or both.
 37. The composition ofclaim 36, wherein the cell population comprises at least 98%cardiomyocyte cells, cardiomyocyte progenitor cells, or both.
 38. Thecomposition of claim 31, wherein the cell population is substantiallydevoid of cells expressing at least one of the following cell surfacemarkers: CD90, CD31, CD140B and CD49A.